1. Field of the Invention
A process and apparatus is provided for the reduction of microorganisms in a conductive medium using low voltage pulsed electrical energy.
2. Description of Related Art
Reduction of microorganisms in a medium using electricity has been studied for many decades. Most early efforts focused on the reduction of microorganisms in a medium by passage of a high voltage electric current through the medium to generate heat, thereby killing the microorganisms in the medium by pasteurization. The conductive medium was often a pumpable food or beverage, such as milk or water.
Later efforts focused on the reduction of the microorganisms by so-called xe2x80x9cnonthermalxe2x80x9d pasteurization methods. These methods involve application of a high voltage electric field to the medium in short pulses. The high voltage electric field generates an applied energy of 150 joules/ml or greater and causes death of the microorganisms by electroporation or lysis of the microbial cell membrane. The shortness of the pulse duration attempted to minimize heating of the medium. However, these methods suffer from numerous disadvantages, especially when applied to pumpable foods and beverages. For example, the high voltage electric field when applied to pumpable foods and beverages can cause structural alterations in the food or beverage, adversely effecting the taste and texture of the food or beverage. In addition, such high applied energies are believed to cause the formation of free radicals in foods and beverages, which compounds are considered to cause or promote cancer. Further, the equipment necessary to generate such high applied energies requires an electrical energy on the order of 100 kV/cm. Furthermore, such methods do not appear to kill all types of microorganisms, such as molds and yeast.
Experiments have been conducted in the prior art using low voltage electric fields. However, these electrical energy applications were considered to be unsatisfactory because they were not deemed to cause irreparable damage to the microorganisms.
As examples of the prior art, reference is made to the following U.S. patents, whose teachings are incorporated by reference: U.S. Pat. No. 4,917,785; U.S. Pat. No. 4,957,606; U.S. Pat. No. 5,026,484; U.S. Pat. No. 5,037,524; U.S. Pat. No. 5,464,513; U.S. Pat. No. 5,514,391; U.S. Pat. No. 5,630,915; U.S. Pat. No. 5,766,447; and the following publications: Bai-Lin Qin et al., xe2x80x9cInactivating Microorganisms Using a Pulsed Electric Field Continuous Treatment Systemxe2x80x9d, IEEE Transactions on Industry Applications, Vol. 34, No. 1, January/February 1998; Karl H. Schoenbach et al., xe2x80x9cThe Effect of Pulsed Electrical Fields On Biological Cellsxe2x80x9d, paper presented at EPRI/Army PEF Workshop II, Chicago, Ill. on Oct. 10-11, 1997; K. H. Schoenbach et al., xe2x80x9cEffect of Pulsed Electric Fields on Micro-organisms: Experiments and Applications, paper presented at EPRI/Army PEF Workshop II, Chicago, Ill. on Oct. 10-11, 1997; and Karl H. Schoenbach et al., xe2x80x9cThe Effect of Pulsed Electric Fields on Biological Cells: Experiments and Applicationsxe2x80x9d, IEEE Transactions on Plasma Science, Vol. 25, No. 2, April 1997.
The process of this invention improves on the prior art by providing a method of microbial reduction in a conductive medium which affects the target microorganism(s) without causing detrimental effects to the medium. The term xe2x80x9creductionxe2x80x9d is used in its conventional sense in the art to mean that the method results in mortality to some or all target organisms. In other words, after treatment with the method of this invention, the treated medium contains a substantially decreased number of viable microorganisms. Applications include conductive mediums such as pumpable foods, beverages, processing fluid streams, blood, water, and eco-system waters, as well as conductive solids and solids suspended in liquids or gases to include air, which mediums are microbiologically infected and capable of causing harm to those consuming or coming in contact with the infected medium. The term xe2x80x9cpumpable foodsxe2x80x9d means any food which is capable of being pumped or conveyed through pipes or conduits, including solid food items conveyed in a conductive aqueous solution. Examples of solid food items in this later category are fruits and vegetables. Conductive solids means any solid item capable of being pumped or conveyed through pipes, conduits or channels. Examples in this category are powderized materials. Suspended solids means any solid suspended in a liquid or gas. Examples in this category are food items in carrier liquids or volatile suspended solids in a wastewater facility.
The method of this invention involves the application of low voltage pulsed electrical energy having defined voltage, frequency and pulse waveform characteristics to the target microorganisms in the medium. By the term xe2x80x9clow voltage pulsed electrical energyxe2x80x9d, it is meant that the combination of energy, frequency and pulse waveform applied to the microorganisms must be such that no free radicals are formed, no ionizing radiation is created, and no osmotic shock waves are formed. The term xe2x80x9clow energy pulsesxe2x80x9d which is used herein by the inventor has the same meaning. It is surprising that the low energy pulses result in cell mortality, since the energy pulses are too low to cause electroporation or lysis of the microbial cell membrane. The specific mechanism by which the method of this invention causes mortality of the microorganisms is not clearly understood. One theory is that the low energy pulses of specific voltage, frequency and pulse waveform cause a disruption in an essential component of the intricate cellular machinery of the microorganism, such as a disruption of the metabolic and/or respiration cycles of the target microorganism. Regardless of exactly how the method of this invention operates to cause mortality to microorganisms, the inventor has demonstrated through extensive experimental tests which are summarized herein that the method is surprisingly effective. Accordingly, when the method is applied with the proper know-how described herein, the ordinary skilled person can achieve substantial reductions in target organisms in a conductive medium by application of low energy pulses which do not have detrimental effects on the medium. As an example of detrimental effects to a medium, there is mentioned the occurrence of organo-leptic changes to a medium which is a pumpable food or beverage.
The effective voltage, pulse frequency and waveform characteristics of each target organism are unique, and therefore the process requires the ability to vary the frequency of energy delivery as well as to vary the voltage applied, with a limitation being such that no combination of applied voltage, pulse frequency and waveform applied is capable of creating structural membrane alterations of the target organisms, e.g., electroporation or lysation of the target organism. Additionally, the combination of energy, pulse frequency and waveform applied must be such that no free radicals are formed, no ionizing radiation is created, nor osmotic shock waves formed. Further, there is substantially no temperature increase or pressure increase.
This process improves on the prior art of disinfection by affecting only the target organism, not the medium. This is accomplished by the controlled release of pulsed energy into a treatment space, such as a conduit or chamber. The process and apparatus may provide for multiple treatment spaces in continuous parallel or series flow paths. The process and apparatus may be installed in a continuous flow production line or in a container, such as a batch storage tank.
The combination of voltage, pulse frequency and pulse waveform are refined such that the energy applied to the target organism disrupts the respiration and/or metabolic codes of the target organism thereby killing the organism. As metabolic and respiration codes are required for living organisms to function, disruption of the codes cause the elimination of the reproductive cycle and death. Surprisingly, the process of the present invention is even capable of killing microorganisms, such as molds and yeast, which are not effected by high voltage electric field methods.
As the voltage, pulse frequency and pulse waveform of the pulsed electrical energy are control parameters, it is preferable to incorporate monitoring with process control into the overall process design to accomplish commercial viability by insuring process consistency, operator safety and documentation of treatment.
Preferably, the parameters to be monitored and controlled which are incorporated into the process design are flow rate of the medium, conductivity of the medium, pH of the medium, pressure of the medium, temperature of the medium, voltage potential between the cathode and anode electrodes of the pulser, current generated by the electrodes into the medium, frequency of the electric pulse, the shape and amplitude of the electric pulse which define the pulse waveform, and the applied energy which is exerted on the microorganisms in the particular medium.
It is specifically noted the process and apparatus described herein are those which represent an improvement in the art. The individual physical components of the apparatus used in the process of the invention, such as pipes, wires, switches, power supplies, pursers, sensors and computers, are currently in existence or can be manufactured by the ordinary skilled artisan using available components. It is the specific way in which these existing components are organized into the apparatus of the invention, and the actual process method for microorganism reduction, that represent the improvement to prior art.
The process method provides for a flow of medium/product to enter and exit a treatment space whereby, while in the treatment space, energy is pulsed into the treatment space via DC electric pulses at a defined voltage, pulse frequency and pulse waveform, which is capable of disrupting the control mechanisms of target organisms. Preferably the flow of medium/product into the treatment space, and its treatment thereof, is a continuous process. Many commercial processes require a continuous flow operation, such as the production process for making a fresh citrus juice. The process and apparatus of this invention are ideally suited for such processes, because the invention may be installed in the continuous flow operation, be used to effectively reduce the naturally occurring microorganisms in the juice, without hindering the speed or arrangement of the normal flow operation. Alternatively, the process is suitable for treatment of microorganisms in a non-continuous flow operation, such as the treatment of medium in a container. For example, control of microorganisms in a batch storage tank is often a problem. The medium contained in the batch storage tank such as juice in a large storage tank or liquids stored at the base of a cooling tower may be circulated through the apparatus of this invention and the microbial content may be reduced using the process of this invention.
The treatment space may be an area of any shape and size which is suitable for holding a conductive medium and subjecting it to low pulsed energy. Preferably the treatment space is defined by the walls of a chamber, the chamber being a partially enclosed space having an inlet and an outlet which are connected to conduits for passing the medium through the chamber for treatment. Within the chamber are at least one pair of electrodes for generating the low pulsed energy. Alternatively, the treatment space may be an area within a conduit itself, such that the treatment space is not enclosed except as defined by the conduit wall and is open to flow of the medium therethrough. The pair of electrodes are inserted through the conduit wall for generating the low pulsed energy within the conduit.
The pair of electrodes are connected by electrical cables to a pulse modulation unit, also referred to herein as a pulser unit or pulser. The pulse modulation unit contains the electrical components, i.e. capacitors, waveform generators, AC to DC transformers, etc., for generating the low voltage electrical pulses, by applying a defined DC voltage to the pair of electrodes, and generating a defined pulse waveform at a defined pulse frequency. Pulse modulation units are commercially available. Preferably the pulse modulation unit is not limited in operation to a single defined voltage, frequency and waveform but is capable of adjustment of these parameters as necessary or desired by the operator. The pulse modulation unit may be proximate or remote from the pair of electrodes. The pulse modulation unit is connected by electrical cables to an AC electrical energy source.
As each family of organisms is different, the metabolic components of the organism, and the information communicated within the organism such as in the form of coded electrical pulses, are also different. Therefore, the process control settings of the invention are required to be variable so as to provide treatment to different organism types within various medium/product.
The process is controlled by a central processing unit (CPU), which may either be a component of the pulse modulation unit or be separate therefrom. The CPU will be programmed to set operating limits for all control parameters. The principle control parameters are pulse frequency, pulse waveform (pulse shape and amplitude) and level of applied voltage.
The pulse frequency may range from 1 to 1000 pulses per second, preferably 60 to 180 pulses per second, more preferably about 120 pulses per second (i.e. 120 Hz).
The pulse waveform is defined by the pulse shape and pulse amplitude. The pulse shape may be any shape, e.g. monopulse, bipulse, bipolar, sine wave, spike, square, etc. Preferably the pulse shape is a monopulse in the positive domain. The pulse amplitude may be in the range of 6,000 V to 15,000 V, more preferably about 12,000 V.
The applied voltage is not limited but may be any suitable voltage which is capable of generating a low voltage pulsed electrical energy into the medium capable of reducing the microorganisms therein, without the formation of free radicals in the medium, without creation of osmotic shock in the medium, and without the generation of ionizing radiation such as lethal UV radiation in the medium.
The amount of applied energy is more critical to the invention than the applied voltage. The applied energy essentially means the amount of energy reaching the target organism in the medium. The applied energy varies based upon the conductivity or resistance of the medium. Thus, a medium having a higher resistance will require a higher voltage in order to generate the same level of applied energy. Furthermore, different organisms are sensitive to different levels of applied energy. Hence a target microorganism must be tested in the medium in which it will be treated to determine the optimum voltage (as well as the optimum frequency and waveform characteristics), and thus the optimum level of applied energy, to kill the organism. For most food and beverage applications, the amount of applied energy must be less than or up to 1 joule/ml. For blood the applied energy would be as minimal as possible to kill the target organisms. The limits of applied energy may be obtained by optimizing the control parameters. For water and other applications, the amount of applied energy is not so limited, but may desirably be so limited if effective against the organism to be killed.
The amount of applied energy is affected by the flow rate of medium/product through the treatment chamber. The flow rate may be in the range of 1 to 300 gallons per minute, preferably in the range of 15 to 25 gallons per minute, more preferably about 20 gallons per minute. For blood treatment the flow rate may be much less than 1 gallon per minute, when the invention is used in similar fashion to a dialysis, or at higher rates when treating large volumes such as those at blood banks.
As noted above, the key aspects of the pulsed electric energy which result in mortality of the microorganism are the applied voltage, frequency and pulse waveform, which subject the target microorganism to a lethal applied energy. Since these key aspects are a function of the medium in which the target organism is contained, it is preferable to monitor and control additional control parameters. Preferably the process and apparatus of the invention monitors and controls the following parameters: 1) flow ratexe2x80x94is related to the number of pulses required to kill an organism type and determines the number of reaction chambers in series required to kill the target organism; 2) conductivityxe2x80x94relates to the ease of pulse travel through the medium; 3) pHxe2x80x94assists in verification the medium has not changed characteristics; 4) pressurexe2x80x94is monitored for verification of consistency in flow rate of medium thereby helping to ensure consistent delivery of uniform energy per unit volume of product; 5) temperaturexe2x80x94is monitored to ensure the applied energy has remained within design limits without raising the temperature of the medium; 6) voltagexe2x80x94the AC power supply is monitored, and DC voltage is monitored across the electrical pulse delivery system for verification of consistent energy delivery; 7) currentxe2x80x94is monitored along with voltage monitoring of the electrical pulse power supply to ensure consistent energy feed conditions for consistent treatment effects; 8) electrical pulse frequencyxe2x80x94is monitored to determine consistent energy per unit volume of medium/product is obtained; 9) pulse shapexe2x80x94is monitored across the electrical pulse delivery unit to ensure consistent treatment.
The control of the process is such that high quality treated medium/product is tantamount. To accomplish this the process monitoring is also control based and interactive. The control system is designed/programmed for application specific environments. That is, each target organism as well as medium/product characteristic is examined to determine the appropriate level of applied energy, frequency and pulse shape necessary to achieve the desired level of microbial reduction in the medium.
It is contemplated that each application may be different and it is anticipated several iterations may be required in pilot studies to refine the final operating conditions. This process allows that flexibility with respect to control range settings of each process variable that is being monitored.
An examination of application installation/location will allow for a determination for the need for redundant or parallel installations of the process. The target organism and medium/product characteristics determine the number of treatment spaces or chambers to be placed in series so as to deliver the correct energy, pulse frequency and pulse shape per unit volume.
The disruption of the metabolic/respiration cycle of the target organism in a specific medium is viewed as a solution couple and must be viewed together. This is a key concept of the invention which is not recognized in the prior art. The amount of applied energy which is effective to achieve mortality of the target microorganism must be determined by measuring the applied energy which achieves mortality for the particular target organism(s) in the particular medium to be treated. Upon determination of optimum control variable ranges, the process control settings are programmed into the CPU.
The control system""s programmable logic controllers are capable of storing operating data. The control system is equipped with an interactive communication modem which allows data stored within to be accessed from a remote location via telephone, cable or satellite links. This will also allow for system diagnostics to be performed from remote locations.
All control variable sensors monitoring flow rate, conductivity, pH, pressure and temperature are preferably on both inlet and outlet locations of the reactor chamber. Voltage, current, frequency of pulse and pulse shape are preferably monitored on the appropriate energy system; voltage and current on the AC and DC systems; frequency of pulse across the pulse delivery system and shape of the pulse off the pulse discharge system.
The PLC units for each sensed/monitored control variable are set to control ranges for each variable.
As one of the process objectives is to create a consistent medium/product, at any time any control variable exceeds a preset limit (e.g. high and low settings for flow rate, conductivity, pH, pressure, temperature) an alarm function (e.g. audible, visual and/or electronic) will activate. At any time current voltage, frequency of pulse, or pulses shape does not meet preset values the same alarm function activates. Alarm activation will result in closing of downstream medium/product conveyance system, opening of a downstream diversion path to a designated storage system for later return back to treatment system; recording of all alarm events; shut down and isolation of medium/product delivery system; perform diagnostics and await operator instructions; auto-dial operator via communication system interface in the event operator is off-site. In the application of blood treatment where blood is shunted from the patient through the treatment space and returned to the patient, any parameter out of scale would shut the treatment space off but allow the blood to continue flowing. The alarm function would alert the attendant of a malfunction.
FIG. 1 shows a process flow schematic which illustrates the sequence of process events in a preferred embodiment of the invention. As each application is most likely different, the medium/product and target organism couple will dictate the number of electric pulse treatment spaces to be placed in series. Applied energy characteristics in terms of voltage, frequency and pulse shape are generally less than or up to one joule/ml for most food and beverage applications to prevent structural changes to the medium/product. Other applications may not be so limited.
The process is adaptive in that a series of electric pulse reactors may be sequenced to achieve microbial reduction without increasing the applied energy per unit volume per pulse.
The process flow path allows for diversion in the event the medium/product does not receive optimum energy application and provides a method of medium/product recovery and retreatment. In-line blood treatment would not allow for diversion of blood. In the event of malfunction, the unit would shut off and alert the attendant.
The process operation is based on a controlled sequence of events. Normal operation passes medium/product through the pretreatment system required by the application; e.g., citrus juice processing would be preceded by culling, grading, disinfection of peel and extraction. The extracted juice would then be pumped through a series of treatment spaces or chambers. If the processing line were large enough or operated continuously, parallel process lines would be used to allow for maintenance and repairs.
The control panel modem interface allows for remote monitoring and control.
On normal operation the applied energy reduces the target organism in the treatment space or chamber and the medium/product is pumped to storage for packaging, etc. If the system senses a fault, the electric valve to the treated medium/product storage tank closes and the electric divert valve to the untreated divert storage tank opens. After repairs, the untreated medium/product is pumped back to the treatment space or chamber.
Parallel operations in multiple production lines allows for continuous maintenance. Parallel production lines can be connected via a common feed header and isolated by electric valves so individual treatment spaces or chambers can be turned on and off via the CPU by establishing a sequencing routine within the control system.
The process described herein is one of microbial reduction of organisms within a conductive medium. The electric pulse process of the invention may be applied to any conductive fluid, conductive solid or suspended solid in a liquid or gas, preferably to pumpable foods and beverages. Other applications of the electric pulse process include for the reduction of micro-organisms in surface waters; for reduction of marine estuary facultative organisms for the purpose of environmental odor control; for reduction of micro-organisms in power plant cooling towers; for the microbiological reduction of volatile solids from wastewater facilities; for reduction of microorganisms in conductive solids such as in powderized materials; and for reduction of microorganisms in a suspended solid such as in solids suspended in a liquid or gas to include air.